Statistical features of 21-cm emission from the epoch between reionization and Gunn-Peterson transparency

We investigate the 21-cm emission from the epoch between reionization zr and Gunn-Peterson transparency zgp. According to the lognormal model of the thermal history around reionization, hydrogen clouds in zr > z > zgp are hot, and a predominant part of baryonic gas is ionized, but still opaque to Lyα photons. Therefore, 21-cm emission is a distinctive characteristic of this epoch. We show that the 21-cm emission comes from both uncollapsed and collapsing hydrogen clouds. The spatial distribution of the brightness temperature excess, δTb, is highly non-Gaussian. It consists of spikes with high δTb and low δTb area between the spikes. The field has the following statistical features: 1.) the one-point distributions of δTb are described approximately by power law tailed probability distribution functions. 2.) The nth-order moment of δTb is increasing much faster with n than that of a Gaussian field, but slower than that of a lognormal field. 3.) The scale-scale correlation of δTb field is significant for all scales larger than the Jeans length of the gas. These features would be useful to distinguish the 21-cm emission of the early clustering from the noise of foreground contamination. Subject headings: cosmology: theory – intergalactic medium – large-scale structure of the universe – methods: statistical